Diene (co)polymers are prepared by polymerization or copolymerization of appropriate monomers in a hydrocarbon solvent in the presence of an organolithium compound and an electron donor additive used to make the copolymerization rate constants of the monomers more consistent, wherein the electron donor additive can be selected from alkali metal alcoholates, tertiary diamines, and ethers [RU 2073023, U.S. Pat. No. 6,867,265 B2, U.S. Pat. No. 4,397,994, U.S. Pat. No. 4,575,534].
Various branching agents providing the formation of branched (co)polymer molecules are used when synthesing rubbers to improve the processability thereof. The degree of branching of (co)polymers has an effect on their properties such as crystallinity, plastic-elastic properties, elasticity of solutions, and melt viscosity, allowing for the creation of new copolymer materials with improved properties.
Analysis of literature sources [F. Tsutsumi, M. Sakakibara, and N. Oshima, Rubber Chem. Technol., 63, 8 (1990); C. A. Siena, C. Galan, M. J. Gomez Eaton, and V. Ruiz Santa Quiteria, Rubber Chem. Technol., 68, 259 (1995)] shows that there are two main groups of branching agents: organosilicon and organotin compounds (silanes, siloxanes, stannates) and vinyl derivatives (divinylbenzene, triacetobenzene, etc.).
The group of organosilicon and organotin compounds is the most often used group. This group encompasses a great diversity of silicon and tin compounds, starting from simplest representatives such as tri- and tetrahalides of silicon and tin, and up to their functionalized alkyl derivatives, such as MeO3Si—(CH2)2—SiMeO3, Cl3Si—(CH2)2—SiCl3, SiCl3—CH2—C(CH2)—CH2—SiCl3, and siloxanes, for example, Cl3Si—O—SiCl3, Cl3Si—O—SiCl2—O—SiCl3.
It should be noted that the simplest representatives of said group of branching agents (SiCl4 or SnCl4) are preferable as a component used for industrial producing SBRs [H. L. Hsieh and R. P. Quirk. Anionic Polymerization: Principles and Practical Applications, Dekker, New York, 1996].
U.S. Pat. No. 4,523,618 patent discloses a method for producing a branched styrene-butadiene rubber by using compounds of the following common formula ClnMeR4-n, wherein n is 3 to 4, Me is Si or Sn, R is Alk, Ar(C1-C20), as a branching agents. The method comprises loading a reactor with monomers, electron donor additives (0.15 to 5% by weight of TGF (tetrahydrofuran) or 0.01 to 0.5% by weight of TMEDA (tetramethylethylenediamine)) and an organolithium initiator. Copolymerization is carried out up to completing conversion of monomers, at 20 to 120° C. for from 0.1 to 24 hours, and thereafter the branching agent is added to the polymerized mixture. After the completion of the branching process, the copolymer is isolated from the reaction mixture by adding lower alcohols. A stabilizer such as 2,6-di-tert-butyl-p-crezole (0.5 to 1% by weight of copolymer) is added to the obtained product. This method makes it possible to produce branched SBRs with a styrene content of 25% and a 1,2-butadiene content of 32%.
The branched copolymers produced by the aforesaid method have a low content of 1,2-butadiene units (not more than 47%), which is insufficient for producing rubbers for treads with a required set of properties, in particular a high wet skid resistance and a low rolling resistance. In order to achieve the aforesaid properties, the optimal content of 1,2-butadiene units in rubber should be more than 60% [Kuperman F. E., Novel rubbers for tires. Priority requirements. Methods of assessment.—Moscow, 2005.—329 p.; Pichugin A. M., Materials science aspects of tire rubber development.—Moscow, 2008.—384 p.].
Patent U.S. Pat. No. 5,066,721 A (Nov. 19, 1991) of Bridgestone Corporation (Japan) also discloses a method for producing a branched styrene-butadiene rubber, wherein organic silicon derivatives containing halogen atoms (chlorine or bromine) are used as a branching agent. The method uses ethers (tetrahydrofuran, dimethoxyethane, and dimethyl and dibutyl ethers of diethylene glycol, etc.) and tertiary amines (pyridine, triethylamine, N,N,N′,N′-tetramethylethylenediamine (TMEDA), N-methylmorpholine, and others) as the electron donor additive. Organolithium compounds, including n-butyllithium, are used as an initiator of the copolymerization process. This method makes it possible to produce copolymers containing about 60% of 1,2-butadiene units.
It is known in the art that the presence of functional groups in the structure of a copolymer, for example tin-, silicon- or amino-containing groups, allows to achieve an improved distribution of reinforcing fillers in the rubber matrix (copolymer), which in turn reduces hysteresis loss and increases the durability and wet grip. [V. R.-S. Quiteria, C. A. Sierra, J. M. Gomez-Fatou, C. Galan, L. M. Fraga. “Tin-coupled styrene-butadiene rubbers (SBRs). Relationship between coupling type and properties”//Macromolecular Materials and Engineering, 1999, 246, 2025-2032; C. A. Uraneck, J. N. Short, “Solution-polymerized rubbers with superior breakdown properties”//J. Appl. Polym. Sci. 2003, 14, 1421-1432].
U.S. Pat. No. 5,268,439 (Dec. 7, 1993) discloses a method for producing a branched functionalized SBR, wherein styrene and butadiene are copolymerized in the presence of an organolithium initiator and electron donor additives, followed by the addition of a branching agent. The lithium initiator is (tributyltin)lithium corresponding to the formula (R)3SnLi, wherein R is alkyl. This initiator acts as a functionalizing agent due to the presence of tin. Compounds such as N,N,N′,N′-tetramethylethylenediamine, tetrahydrofuran, etc. can be used as the electron donor additive. As the coupling branching agent compounds are used that are selected from SnCl4, alkyl-tin chlorides, and N,N′-dimethylethylurea. The resultant copolymer of general formula R3SnYLi, wherein Y is a copolymer radical, has a styrene content of 20.6%, a vinyl content of 51.4%; a Mooney viscosity of 65; tear strength of 21 MPa, relative elongation of 376%; tgδ (23° C.) of 0.1079; tgδ (50° C.) of 0.0739; Mn=556000 g/mol.
This method provides copolymers with a low content of 1,2-butadiene units and a high value of polydispersity. In addition, a disadvantage of this method consists in the necessity to use an expensive and unstable lithium initiator that preparation takes about 20 hours.
U.S. Pat. No. 5,959,048 (Sep. 28, 1999) considered as the closest prior art discloses a method of forming a branched functionalized styrene-butadiene rubber in an acyclic alkane solvent and in the presence of an amine-containing organolithium initiator, wherein the initiator is a mixture of a lithium amine of formula A1Li taken in an amount of about 90 to about 10 parts by weight and lithium amine of formula A2Li taken in an amount of about 10 to about 90 parts by weight, wherein A1 and A2 are different or independently selected from the group consisting of dialkyl-, alkyl-, cycloalkyl- and dicycloalkylamine radicals of the formula

and cyclic amine radicals of the formula

wherein each R1 is independently selected from the group consisting of alkyl, cycloalkyl and aryl, having from 3 to about 12 carbon atoms, and R2 is selected from the group consisting of an alkylene, oxy- or amino-alkylene group having from about 3 to about 16 methylene groups (for example, a mixture of lithium trimethyl-hexamethyleneamide and lithium 3,3,5-trimethyltetrahydroazepine). Then the obtained polymer reacts with a branching agent, which may be, in particular, a compound of the formula (R3)aZXb, wherein Z is tin or silicon, R3 is selected from the group consisting of alkyl comprising from 1 to 20 carbon atoms, aryl comprising from 6 to 20 carbon atoms; X is chlorine or bromine; a is 0 to 3, b is 1 to 4, and a+b=4. The initiator used in this known method is prepared by a reaction of n-butyllithium with a mixture of brenched amines of the above-indicated structure in an alyphatic solvent medium. Compounds useful as the electron donor additive are tetrahydrofuran, 2-2′-ditetrahydrofuranpropan, dipiperidinethane, dimethyl ether, diethyl ether, tributylamine, tetramethylethylenediamine (TMEDA), ethylene glycol ethers, and crown ethers. The resulting polymer is vulcanized to produce rubbers with reduced hysteretic loss, reduced rolling resistance and reduced heat build-up. The resulting copolymer contains 20.1% of styrene (1.6% of block styrene), 43.8% of 1,2-butadiene units, and has glass transition temperature of −49° C., Mn=145511, polydispersity of 1.90, and a Mooney viscosity of 62.
Copolymers produced according to this method have a low content of 1,2-butadiene units. In addition, a disadvantage of the method of forming a functionalized polymer consists in the necessity of using a solution of unstable lithium amides as an initiator, wherein precipitation of the nitrogen-containing initiator from the solution occurs after 30-day storage. Such precipitation decreases the concentration of the initiator in the solution and makes it difficult to deliver the initiator in an amount required for the polymerization. In addition, the initiator is obtained by a reaction of n-butyllithium with a mixture of two different amines since the use of only one of amines results to its immediate precipitation [RU Patent 2175330].
In this connection, the purpose of the present invention is to develop an effective method for producing branched functionalized diene (co)polymers characterized by a statistical distribution of monomer units, high content of vinyl units (1,2-butadiene units and/or 3,4-isoprene units (more than 60%)), and a narrow range of the molecular mass distribution (1.4-1.7). The method allows regulation of the amount of the branched and functionalized parts in the (co)polymer.